1. Field of the Invention
The present invention concerns an oxide semiconductor target, an oxide semiconductor material; and a thin film transistor (TFT) having a semiconductor channel layer formed by using the oxide semiconductor target.
2. Description of the Related Art
Since thin film transistors (TFT) have a small device area and thus are space-saving, they are used as transistors for driving display devices in various portable electronic equipment such as mobile telephones, notebook personal computers, and PDAs.
Heretofore, most of thin film transistors have been manufactured from silicon semiconductor materials typically represented by amorphous silicon or polycrystal silicon. This is because the thin film transistors can be manufactured by using conventional manufacturing steps and manufacturing techniques of semiconductor devices.
However, when the thin film transistors are manufactured by using the conventional steps of manufacturing semiconductor devices, since the processing temperature is 350° C. or higher, substrate materials usable therefor are restricted. Particularly, since heat resistant temperature of most of glass substrates or flexible resin substrates is 350° C. or lower, it is difficult to form the thin film transistors on the substrates described above by using the conventional steps of manufacturing the semiconductor devices.
Accordingly, research and development have been conducted for thin film transistors using oxide semiconductor materials that can be deposited at a low temperature for a channel layer. By using the oxide semiconductor film to the channel layer, thin film transistors can be formed over the glass substrates or flexible resin substrates. Further, since oxide semiconductors have a field-effect mobility higher by one digit or more than that of the conventional amorphous Si, a novel device not present so far can be manufactured at a low cost. Accordingly, many applications of using the oxide semiconductor materials have been reported for RFID (Radio Frequency Identification) tags as well as display devices.
Among them, for obtaining an OLED (Organic Light-Emitting Diode) expected as a display device in the next generation, it is necessary and indispensable to ensure the field-effect mobility of 5 cm2/Vs or more and the reliability to light (stress resistance) since the OLED is a self-emitting device. Further, with a view point of power consumption, it is important to obtain a TFT having a small subthreshold slope (subthreshold swing).
As the material for the oxide semiconductor, In—Ga—Zn—O (IGZO), etc. have been reported. However, since the materials contain rare metals such as In and Ga in a great amount, there may be a concern of increasing the material cost. On the contrary, since Zn—Sn—O (ZTO) materials contain no rare metals, they are superior in view of resource and cost. For improving the reliability of ZTO, Japanese Patent Laid-open No. 2012-033854 reports suppression of plasma damage during formation of a protective film and decrease of a threshold voltage shift amount (ΔVth=−4 to −6.5 V) in a light stress test, by adding at least one of Al, Hf, Ta, Ti, Mb, Mg, and Ga to ZTO by 0.01 to 0.3 at %. However, the threshold voltage shift amount after formation of the protective film is 4 V in terms of an absolute value and improvement is necessary for the reliability and the controllability of the threshold voltage (Vth). Further, Japanese Patent Laid-Open No. 2011-205054 reports characteristics of a semiconductor synthesized at a ratio of starting materials of Zn:Sn:Hf=0.003 mol:0.003 mol:0.0012 mol with an aim of improving the electric characteristics of a coated type semiconductor material. While improvement of the semiconductor characteristics has been conformed by addition of about 17% by mol of Hf to ZTO in the coated type semiconductor material, there are still present many problems in the reliability (stress resistance).
In view of the above, for applying ZTO oxide semiconductor materials to the OLED described device, etc., it is necessary to satisfy the field-effect mobility of 5 cm2/Vs that is necessary for driving the OLED and a low threshold voltage that can control the threshold voltage. For this purpose, the improvement of the field-effect mobility and the improvement of the stress resistance against light or electricity are called for.